It sounds like a POOR fitting breach plug more than anything else, like a over charge. That, and someone correct me if I'm wrong, but 777 is not a BP substitute, grain per grain. It burns with more pressure and velocity than a equal amount of BP. The more I think of it the more have have trouble I have believing the story about the TC. I don't think it would matter how much BP you put down the barrel - with only one projectile the breech plug would not be injured.
I started muzzle loading back around 1971. Back then it was hammer guns and flintlocks and real BP. People did it because they liked to to things they it was done a 100/200 years ago. Now its morphed into modern crap for powders and inline guns. Most guys just do it to shoot another deer or extend their hunting season. Now it's just putting a couple of pellets down the barrel. We use to get a new gun and go to the target bench for a couple of hours of trying different loads for the best group with a patched round ball. You were reliving history. And you bought and read real books about it. It was fun. I feel sorry for all you guys missing out on what could be some very pleasurably hours spent shooting real BP guns. Just something to think about if you're not in too much of a hurry. Good shooting to all and kept your powder dry.